The present invention relates to a semiconductor memory capable of measuring a threshold voltage in a memory cell, more particularly to an electrically erasable and programmable read only memory (hereinafter referred to as EEPROM) capable of measuring the threshold voltage in the memory cell in which a data "1" and a data "0" (hereinafter referred to simply as "1" or "0") are written.
In a screening process of the EEPROM. it is important to measure the threshold voltage of the EEPROM element constituting the memory cell.
The EEPROM has a source, a drain, a floating gate and
a control gate wherein "1" or "0" is written therein by charging positively or negatively. When the data is read out from the EEPROM, a predetermined voltage is applied to the control gate. Assume that "0" is written in the EEPROM if a current which is greater than the threshold current (It) flows between the source and the drain and "1" is written in the EEPROM if a current which is less than the threshold current between the source and the drain.
When the voltage to be applied to the control gate is increased, the current to flow between the source and the drain is proportionally increased.
The threshold voltage is defined as the voltage to be applied to the control gate of the EEPROM element in which "1" is written when the same voltage is increased from zero volt so that the current to flow between the source and the drain reaches the threshold current (It).
It is possible as evident from the above explanation to measure the threshold voltage of the EEPROM element in which "1" is written. However, it is impossible to measure the threshold voltage of the EEPROM element in which "0" is written.